Data and code from: Western larch regeneration more sensitive to wildfire-related factors than seasonal climate variability

To understand the impacts of changing climate and wildfire activity on
conifer forests, we studied how wildfire and seasonal post-fire climate
conditions influence western larch (Larix occidentalis) regeneration
across its range in the northwestern US. We destructively sampled 1651
seedlings from 57 sites within 32 fires that burned at moderate or high
severity between 2000 and 2015, and which were within 100 m of
reproductively mature western larch. Using dendrochronological methods, we
estimated germination years of seedlings to calculate annual recruitment
rates. We used boosted regression trees to model the annual probability of
recruitment as a function of (i) “wildfire-related factors” including
distance-to-seed-source, satellite-derived fire severity, and
time-since-fire, and (ii) seasonal climate conditions, including variables
reflecting temperature and water availability. Most recruitment occurred
within five years after wildfire, at sites within 25 m of reproductively
mature western larch. Wildfire-related factors had the highest relative
influence on post-fire recruitment (87%), whereas post-fire seasonal
climate had less influence (13%). Annual recruitment probability increased
with growing season actual evapotranspiration, to a maximum c. 275 mm, and
then decreased. Annual recruitment probability decreased as growing season
climatic water deficit increased. Our results suggest that recent climate
trends – increased growing season water deficit and decreased actual
evapotranspiration – have had variable, yet net-neutral, impacts on the
climate suitability for post-fire western larch regeneration across its
range. Climate suitability increased modestly at ‘cooler-and-wetter’ sites
and decreasing modestly at ‘warmer-and-drier’ sites. The strong influence
of wildfire-related factors highlights the potential for management
decisions to promote western larch. For instance, facilitating prescribed
or managed wildfire with moderate- to high-severity patches will generate
conditions suitable for natural regeneration, provided sufficient seed
sources. Additionally, our findings support monitoring for natural
regeneration or directing outcomes by planting within the first five years
after fire, consistent with current management practices.

为探究气候变化与野火活动对针叶林的影响，我们研究了野火及火后季节气候条件如何影响西部落叶松（Larix occidentalis）在其美国西北部分布区内的再生过程。我们对2000-2015年间发生的32场中高烈度野火中的57个样点进行了破坏性采样，共采集1651株幼苗，且这些样点均位于生殖成熟西部落叶松100米范围内。采用树轮年代学（dendrochronology）方法，我们估算了幼苗的发芽年份，以计算年更新率（annual recruitment rate）。我们采用增强回归树（boosted regression tree）构建模型，将年更新概率作为以下两类因子的函数：（i）野火相关因子，包括种子源距离、卫星反演火烈度及火后时间；（ii）季节气候条件，包括反映温度与水分可利用性的变量。大多数更新发生在野火后的五年内，且位于生殖成熟西部落叶松25米范围内。野火相关因子对火后更新的相对影响最大（87%），而火后季节气候的影响较小（13%）。年更新概率随生长季实际蒸散量（actual evapotranspiration）增加而上升，直至约275毫米时达到峰值，随后下降；随生长季气候水分亏缺（climatic water deficit）增加而下降。我们的结果表明，近期气候趋势（生长季水分亏缺增加及实际蒸散量减少）对西部落叶松在其分布区内火后再生的气候适宜性产生了可变但净中性的影响：在“凉爽湿润”样点，气候适宜性略有提升；而在“温暖干燥”样点，则略有下降。野火相关因子的显著影响凸显了管理决策在促进西部落叶松生长方面的潜力。例如，若存在充足种子源，促进具有中高烈度斑块的计划烧除或管理野火，将创造适合自然再生的条件。此外，我们的研究结果支持在火后前五年内监测自然再生或通过种植引导结果，这与当前管理实践一致。